src/ filenames now consistently lowercased

[eddelbuettel]

This is entirely cosmetic: three remaining uppercase-named files in src/ are now lower-cased so that the compilation looks a little prettier:

[... also edited slightly below ...]
** libs
g++ -I[...]  -c api.cpp -o api.o         
g++ -I[...]  -c attributes.cpp -o attributes.o        
g++ -I[...]  -c barrier.cpp -o barrier.o  
g++ -I[...]  -c date.cpp -o date.o         
g++ -I[...]  -c module.cpp -o module.o        
g++ -I[...]  -c rcpp_init.cpp -o rcpp_init.o     
g++ -shared -o Rcpp.so api.o attributes.o barrier.o date.o module.o rcpp_init.o \
    -L/usr/lib/R/lib -lR                                    
installing to /usr/local/lib/R/site-library/Rcpp/libs
[...]

[eddelbuettel]

@kevinushey @jjallaire if you have a split-second to glance at this I'd appreciate it.

Should be uncontroversional apart from the implicit bike-shedding for which I apologize 😃

[kevinushey]

LGTM!